The term “cement”, which come from the Latin meaning “crushed stone”, refers to a material of binding one substance to another substance.
Hydraulic cement means cement of forming a water-proofing product, a representative example thereof is Portland cement which is produced by grinding clinker, which is obtained by mixing limestone with clay at a proper mixing ratio and then heating the mixture to a temperature of 1,450° C. to 1,550° C. using a rotary kiln, with about 5 wt % of gypsum (calcium sulfate) to a size of 75 μm or less using a ball mill to control hydraulic setting reactions, and examples thereof include blast furnace slag cement containing 30 wt % to 40 wt % of a blast furnace slag with Portland cement, pozzolan cement containing 20 to 25 wt % pozzolan with Portland cement, expansive cement containing an expandable additive with Portland cement, quick-setting/rapid-hardening cements such as blends of calcium aluminate cement with Portland cement, and blends of Portland cement with plaster, or fine particle-type Portland cement, oil-well cement obtained by reducing the amount of aluminum oxide present in Portland cement and conducting rough crushing, white cement produced by reducing an iron content of Portland cement, modified cement such as colored cement produced by adding a pigment such as iron oxide, chromium oxide or cobalt blue to white cement and calcium aluminate cement used to produce cold weather concrete and refractories.
The hydraulic cement produces a waterproof hard cured substance, when mixed with water, to cause a chemical reaction producing hydrates. This process may be classified into (1) losing flowability in the physical view, (2) setting inducing solidification of a plastic cement paste and (3) curing.
Portland cement is utilized, as a starting material, in the form of a gray powder composed of coarse particles with a size of 1 μm to 50 μm and, when the gray powder is dispersed in water, hot products of calcium sulfate and calcium contained in the cement are readily dissolved to produce a variety of ions. First, these ions produce an acicular crystal, ettringite, and then calcium hydroxide prismatic crystals and thin acicular crystals of calcium silicate hydrate fill the spaces where the cement particles are dissolved in water. Accordingly, it is known that the cement paste losses flowability (1), is set and unstable ettringite is degraded into calcium alumina silicate hydrates taking the form of a stable hexagonal plate-type crystal again (2) and is then cured (3).
As such, a flowable cement paste can be produced by simply mixing hydraulic cement with water and can be utilized in a variety of applications including mortar obtained by adding sand to the flowable cement paste, concrete obtained by adding sand and aggregate to the flowable cement paste, grout obtained by adding fine aggregate thereto, shotcrete for spraying and the like, and then widely used in the form of composite materials in construction, civil engineering and art installations after curing.
Meanwhile, it is known that the cured cement paste contains a large number of pores having various sizes and irregular shapes (1) and crystal products produced by hydration of the hydraulic cement are also very irregular (2). In general, a cured substance produced using hydraulic cement has a low compressive strength of 7% to 11% in terms of tensile strength due to presence of pores and structural irregularity. However, temperature and humidity changes resulting from climate changes in natural environments frequently and repeatedly cause expansion and shrinkage depending on temperature, and expansion and shrinkage depending on water proportion changes. The cured cement paste has a major drawback in that it receives tensile strength when shrinkage is induced, and thus cracks and is readily deteriorated. The low tensile strength of the cured hydraulic cement is a major obstacle limiting the application scope of cement.
Factors such as air entrainment, curing period and conditions and size of molded materials affect formation of the pores in the cured cement, but it is known that the amount of water added in the process of producing cement paste is most closely related to formation of the pores in the cured cement, and cured cement paste produced using a large amount of water has a higher pore content and much lower strength than a cured cement paste produced using a smaller amount of water.
Active research and development to solve these drawbacks of the hydraulic cement has been underway and many technologies related thereto have been published. The research and development may be broadly classified into: (1) adding fibrous reinforcement materials such as asbestos, glass fiber, metal fiber, ceramic fiber, natural fiber and synthetic fiber; (2) producing cement paste containing a small amount of water and applying mechanical compression or vibration for firming prior to curing; (3) developing an admixture for controlling surface activity of cement particles, and setting and applying the same; and (4) adding liquid or granular plastic additives.
Recently, addition of liquid or granular plastic additives to cement paste has received attention as a method of fundamentally solving drawbacks of water-curable cured cement because it improves flowability of hydraulic cement, thereby reducing use of mixed water and the cured cement contains plastic additives uniformly present therein, thereby improving adhesion between two cured cements, or between an aggregate and cured cement.
For example, U.S. Pat. No. 3,951,674 points out that cements need excess water to improve workability and this causes a deterioration in strength of the cured cement, and suggests addition of 0.3 wt % to 2 wt % of water-soluble cellulose acetate sulfate, as an alternative therefor, with respect to the weight of Portland cement so as to delay curing time of cement paste (1) and reduce friction between cement and aggregated particles, thereby improving workability with a small amount of water and obtaining a cured substance with excellent strength (2).
In addition, U.S. Pat. No. 4,880,467 argues that cured cement should have a flexural strength of 15 MPa or more, preferably 40 MPa or more, so that the cured cement has sufficient durability to withstand outer environmental changes and discloses that this can be accomplished by, as an uncured cement particle paste containing particles with a size of less than 100 μm, a cement paste which contains one or more hydraulic cements and 1 to 20 parts by weight of a synthetic resin selected from a styrene-butadiene copolymer, an acrylester polymer, a vinyl acetate polymer, a vinylidene chloride polymer, an epoxy resin, a phenol resin, a urethane resin and an acrylic resin, with respect to 100 parts by weight of hydraulic cement, and further contains 8 to 20 parts by weight of water with respect to 100 parts by weight of the hydraulic cement.
As such, there are a variety of plastics applicable upon addition of the liquid or granular plastic additive to the cement paste and technologies for applying a number of types of plastic materials to cement have been suggested to date. However, in order for plastic additives to be mixed to cement paste to reduce voids of cured cement, offer an additional adhesive force between particles and improve tensile strength of cured substance, first of all, affinity between the plastic additive and cured cement should be considered.
Polyurethane, which is a plastic having a urethane bond, is produced by reaction of an isocyanate compound with polyol, and research is widely underway to apply polyurethane to cured cement because the cured cement has strength, abrasion resistance, oil resistance and elasticity, in particular, strong adhesion strength with cured cement.
For example, Korean Patent No. 0892247 discloses a polyurethane-based cement composition which is prepared by mixing 50 to 1,000 parts by weight of dry mortar with 100 parts by weight of a resin mixture consisting of a main ingredient and a curing agent in a ratio of 1:1 and is applicable as a waterproof coating material, a coating material or a substrate controller when applied to a substrate such as asphalt or concrete.
In addition, Korean Patent No. 1135593 discloses a composite with excellent sound-proofing property and heat insulation which is prepared by mixing 50 to 70 parts by weight of a cement powder with 10 to 30 parts by weight of a powder additive to prepare a cement mixture powder, mixing a mixture of isocyanate, polyol and a foaming agent with the cement mixture powder prepared above and then conducting foaming under pressure.
In another example, Korean Patent No. 1075260 discloses a urethane resin mortar composition for ground paving which is prepared by mixing 150 to 200 parts by weight of cement, 25 to 75 parts by weight of an inorganic pigment, 25 to 75 parts by weight of an anti-sedimentation agent and 25 to 75 parts by weight of a self-leveling agent, with respect to 100 parts by weight of a liquid acrylic urethane resin.
However, all of these patents relate to technologies associate with combination of substances such as cement and urethane, or cement, urethane, water and the like, or use of a new application or processing method, suggest only a method for manufacturing a mix composition obtained in the form of a uniform liquid, or a method of applying a liquid composition obtained in the form of a uniform liquid, and do not teach technologies which include discharging and separating free water present between cement particles via a process of foaming polyurethane in a cement paste, although sufficient blended water to uniformly hydrate cement particles is added, to provide a flowable mixture composition of polyurethane and cement paste, containing a very small amount of water.